One non-deal behavior of transistors is the so-called “spurious turn-off” and spurious turn-on” effects. These effects refer to an unintended switching behavior caused by negative voltage feedback on the gate during a switching event. Spurious turn-off occurs during a switch ON operation. Although the device receives a positive voltage (in the case of a positive threshold device) that is intended to turn the device ON, feedback produced by the switching operation lowers the voltage at the gate. If the feedback is large enough, this negative voltage will drop the gate voltage below the threshold of the device and induce a turn OFF operation, i.e., cause the opposite of what is intended. A symmetrical effect occurs during a switch from ON to OFF, i.e., the device momentarily turns back ON.
Spurious turn-off and turn-on can occur in high power applications, e.g., applications that require switching of large voltages, such as 200V, 400V or more as well as medium or low power applications, e.g., applications that require switching of 20V or less. In high power applications, the relatively large voltage that appears at the output terminals (e.g., drain-source terminals) of the transistor will rapidly decrease during a turn ON operation, and vice-versa. Thus, a large dv/dt signal will appear at the output terminals of the transistor. The CGS (gate source capacitance) and the CGD (gate drain capacitance) of the transistor appear as a capacitive voltage divider to this dv/dt. As a result, the gate capacitor of the transistor charges.
Power HEMTs (high-electron-mobility transistors) are generally preferred in power switching applications due to their favorable power density, on-state resistance, switching frequency, and efficiency benefits over silicon MOSFETs, for example. An HEMT is a transistor with a heterojunction between two materials having different band gaps, such as GaN and AlGaN. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises near the interface between the AlGaN barrier layer and the GaN buffer layer. In an HEMT, the 2DEG forms the channel of the device. Without further measures, the heterojunction configuration leads to a self-conducting, i.e., normally-on, transistor. A variety of solutions exist to modify this normally-on configuration into a normally-off device. For example, p-type GaN material can be incorporated into the gate structure of the HEMT to make the device a normally-off device.
Spurious turn-off and turn-on is especially difficult to control in HEMT devices. In general, an increase in CGS or Vth (threshold voltage) will mitigate the problem, as the device can absorb more charge before reaching the threshold. This is difficult to achieve and/or costly in HEMTs due to the design of the gate structure. The problem can also be addressed by through design of the gate driver circuitry. However, in many applications, the driver circuitry is provided externally. In that case, parasitic inductances and capacitances that appear between the driver circuitry and the power transistor may make it difficult or impossible to rapidly dissipate charges from the gate of the device.